This invention relates to the production of regular grade cube-on-edge oriented silicon steel strip and sheet of less than 0.30 mm thickness by a simplified process. More particularly, the process of the invention omits an anneal of the hot rolled material with consequent saving in energy costs and processing time, without sacrificing the magnetic properties. This is made possible by conducting an anneal of the cold rolled strip at intermediate thickness at a higher temperature than that of a conventional intermediate anneal.
The so-called "regular grade" silicon steel having the cube-on-edge orientation utilizes manganese and sulfur (and/or selenium) as a grain growth inhibitor. In contrast to this, "high permeability" silicon steel relies upon aluminum nitrides in addition to or in place of manganese sulfides and/or selenides as a grain growth inhibitor.
The process of the present invention is applicable only to regular grade grain oriented silicon steel, and hence purposeful aluminum and nitrogen additions are not utilized.
The conventional processing of regular grade grain oriented silicon steel strip and sheet comprises the steps of preparing a melt of silicon steel in conventional facilities, refining and casting in the form of ingots or strand cast slabs. The cast steel preferably contains, in weight percent, from about 0.02% to 0.045% carbon, about 0.04% to 0.08% manganese, about 0.015% to 0.025% sulfur and/or selenium, about 3% to 3.5% silicon, not more than about 50 ppm nitrogen, not more than about 30 ppm total aluminum, and balance essentially iron.
If cast into ingots, the steel is conventionally hot rolled into slabs. The slabs (whether obtained from ingots or continuously cast) are heated (or reheated) to a temperature of about 1300.degree. to 1400.degree. C. in order to dissolve the grain growth inhibitor prior to hot rolling, as disclosed in United States Pat. No. 2,599,340. The slabs are then hot rolled, annealed, cold rolled in two stages with an intermediate anneal, decarburized, coated with an annealing separator and subjected to a final anneal in order to effect secondary recrystallization.
Representative processes for producing regular grade cube-on-edge oriented silicon steel strip and sheet are disclosed in United States Pat. Nos. 4,202,711; 3,764,406; and 3,843,422.
The process of U.S. Pat. No. 4,202,711 includes hot rolling of a strand cast slab with a finish temperature greater than 900.degree. C., an anneal of the hot band at 925.degree. to 1050.degree. C., pickling, cold rolling in two stages with an intermediate anneal within the temperature range of 850.degree. to 950.degree. C. and preferably at about 925.degree. C. with a soak time of about 30 to 60 seconds. The material is then cold rolled to final thickness, decarburized, coated with an annealing separator and finally annealed in a hydrogen-containing atmosphere.
United States Pat. No. 2,867,558 discloses a process for producing cube-on-edge oriented silicon-iron wherein a hot reduced silicon-iron band containing more than 0.012% sulfur is cold reduced at least 40%, subjected to an intermediate anneal between 700.degree. and 1000.degree. C. to control the average grain size between about 0.010 and about 0.030 mm, further cold reduced at least 40% to final thickness, and finally annealed at a temperature of at least 900.degree. C. It was alleged that excessive grain growth occurred at intermediate annealing temperatures above 945.degree. C. unless relatively large amounts of sulfur and manganese (or titanium) were present in the silicon-iron. Thus, a sulfur content of 0.046% and a manganese content of 0.110% were required in order to avoid a grain size in excess of 0.030 mm when annealing at 975.degree. C. for 15 minutes.
United States Pat. No. 2,867,559 discloses the effect of intermediate annealing time and temperature on grain size and percent of cube-on-edge orientation for a single composition selected from U.S. Pat. No. 2,867,558, containing 3.22% silicon, 0.052% manganese, 0.015% sulfur, 0.024% carbon, 0.076% copper, 0.054% nickel, and balance iron and incidental impurities. The intermediate annealing temperature disclosed in this patent ranged from 700.degree. to 1000.degree. C. and the total annealing times of 5 minutes or more.
United States Pat. No. 4,212,689 discloses that nitrogen should be decreased to a low level of not more than 0.0045% and preferably not more than 0.0025% in order to achieve a very high degree of grain orientation. The process involves an initial anneal of hot rolled silicon steel at 950.degree. C., cold rolling to intermediate thickness, conducting an intermediate anneal at 900.degree. C. for 10 minutes, and further processing in conventional manner except for an additional final annealing treatment.
Other patents of which applicant is aware include U.S. Pat. Nos. 3,872,704; 3,908,737 and 4,006,044.